﻿using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace org.webguvenligi.NSecureImageLib.GraphicFileFormats.CRC
{
    /// <summary>
    /// Computes Adler32 checksum for a stream of data. An Adler32
    /// checksum is not as reliable as a CRC32 checksum, but a lot faster to
    /// compute.
    /// 
    /// The specification for Adler32 may be found in RFC 1950.
    /// ZLIB Compressed Data Format Specification version 3.3)
    /// 
    /// 
    /// From that document:
    /// 
    ///      "ADLER32 (Adler-32 checksum)
    ///       This contains a checksum value of the uncompressed data
    ///       (excluding any dictionary data) computed according to Adler-32
    ///       algorithm. This algorithm is a 32-bit extension and improvement
    ///       of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073
    ///       standard.
    /// 
    ///       Adler-32 is composed of two sums accumulated per byte: s1 is
    ///       the sum of all bytes, s2 is the sum of all s1 values. Both sums
    ///       are done modulo 65521. s1 is initialized to 1, s2 to zero.  The
    ///       Adler-32 checksum is stored as s2*65536 + s1 in most-
    ///       significant-byte first (network) order."
    /// 
    ///  "8.2. The Adler-32 algorithm
    /// 
    ///    The Adler-32 algorithm is much faster than the CRC32 algorithm yet
    ///    still provides an extremely low probability of undetected errors.
    /// 
    ///    The modulo on unsigned long accumulators can be delayed for 5552
    ///    bytes, so the modulo operation time is negligible.  If the bytes
    ///    are a, b, c, the second sum is 3a + 2b + c + 3, and so is position
    ///    and order sensitive, unlike the first sum, which is just a
    ///    checksum.  That 65521 is prime is important to avoid a possible
    ///    large class of two-byte errors that leave the check unchanged.
    ///    (The Fletcher checksum uses 255, which is not prime and which also
    ///    makes the Fletcher check insensitive to single byte changes 0 -
    ///    255.)
    /// 
    ///    The sum s1 is initialized to 1 instead of zero to make the length
    ///    of the sequence part of s2, so that the length does not have to be
    ///    checked separately. (Any sequence of zeroes has a Fletcher
    ///    checksum of zero.)"
    /// </summary>
    public class Adler32Hash : System.Security.Cryptography.HashAlgorithm
    {
        /// <summary>
        /// largest prime smaller than 65536
        /// </summary>
        private const uint BASE = 65521;
        
        private uint checksum;

        protected override void HashCore(byte[] array, int ibStart, int cbSize)
        {
            if (array == null)
            {
                throw new ArgumentNullException("buffer");
            }
            if (array == null)
            {
                throw new ArgumentNullException("buffer");
            }

            if (ibStart < 0)
            {
#if NETCF_1_0
				throw new ArgumentOutOfRangeException("offset");
#else
                throw new ArgumentOutOfRangeException("offset", "cannot be negative");
#endif
            }

            if (cbSize < 0)
            {
#if NETCF_1_0
				throw new ArgumentOutOfRangeException("count");
#else
                throw new ArgumentOutOfRangeException("count", "cannot be negative");
#endif
            }

            if (ibStart >= array.Length)
            {
#if NETCF_1_0
				throw new ArgumentOutOfRangeException("offset");
#else
                throw new ArgumentOutOfRangeException("offset", "not a valid index into buffer");
#endif
            }

            if (ibStart + cbSize > array.Length)
            {
#if NETCF_1_0
				throw new ArgumentOutOfRangeException("count");
#else
                throw new ArgumentOutOfRangeException("count", "exceeds buffer size");
#endif
            }

            //(By Per Bothner)
            uint s1 = checksum & 0xFFFF;
            uint s2 = checksum >> 16;

            while (cbSize > 0)
            {
                // We can defer the modulo operation:
                // s1 maximally grows from 65521 to 65521 + 255 * 3800
                // s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31
                int n = 3800;
                if (n > cbSize)
                {
                    n = cbSize;
                }
                cbSize -= n;
                while (--n >= 0)
                {
                    s1 = s1 + (uint)(array[ibStart++] & 0xff);
                    s2 = s2 + s1;
                }
                s1 %= BASE;
                s2 %= BASE;
            }

            checksum = (s2 << 16) | s1;
        }
        
        protected override byte[] HashFinal()
        {
            return BitConverter.GetBytes(checksum);
        }

        public override void Initialize()
        {
            checksum = 1;
        }
    }   
}
